1. Field of the Invention
This invention relates to boundary lubricants for metal and ceramic wear surfaces such as those used in close tolerance hydrodynamic bearings, cams, or sliding-on-wear surfaces.
2. Description of the Prior Art
Metal or ceramic wear surfaces such as iron, steel, chromium, nickel, silicon, quartz and sapphire, for applications such as gyroscope spin bearing parts, rolling-and-sliding element bearings, machine parts, and cams impose stringent requirements on boundary lubricants used to reduce wear and friction between the surfaces.
Present methods of lubrication do not combine durable wear and friction resistance characteristics. For example, oil-type lubricants, which are not true boundary lubricants, move relative to the wear surfaces, decompose under the resulting frictional contact, and provide a vehicle for the agglomeration of metal wear particles.
Classic boundary lubricants include stearates, such as heavy metal or sodium stearate, and simple fatty acids, such as stearic acid (CH.sub.3 (CH.sub.2).sub.16 COOH). (The use of fatty acids and metal salts of fatty acids, such as Cu or Ag stearates, as boundary lubricants is set forth by F. P. Bowden and D. Tabor in The Friction and Lubrication of Solids, at pp 176-227 (Oxford Univ. Press, London, 1950)). The classic boundary lubricants have a polar head such as carboxyl (COOH) that is characterized by high electron density and high chemical activity and that readily aligns itself on other polar sites. Considering iron wear surfaces for purposes of illustration, the polar head anchors itself to the Fe--FeO thereon. A long chain hydrophobic tail portion, R, of the boundary lubricant takes up shear stress between opposing wear surfaces, lowering the coefficient of friction and decreasing wear. However, the classic boundary lubricants typically are fusible. Many classic lubricants have melting points in the range of 80.degree.-250.degree.C and, when melted, do not have sufficiently strong intermolecular forces of attraction, so that the lubricants tend to migrate and the boundary load limit is markedly reduced.
Accordingly, it may be appreciated that there exists a need for a durable boundary lubricant that imparts a low coefficient of friction and superior wear characteristics to metal and ceramic wear surfaces, is non-melting, and is non-migrating.
Critical considerations for wear surfaces in applications such as gas-lubricated hydrodynamic bearings include the low hydrodynamic lift and the low viscosity provided by the hydrodynamic lubrication. At low angular speeds (for example, during starting and stopping), hydrodynamic lift is frequently inadequate to separate the rotating bearing members from the stationary members. The low viscosity of the gaseous working fluids requires that bearings have very small clearances in order to provide useful load capacity and stiffness. For example, a typical sleeve and journal cylindrical bearing of 1/4 inch diameter may be limited to an operating radial gap of no more than 40 .times. 10.sup.-.sup.6 inches.
To provide good dimensional control and to withstand the moving contact that occurs during starting and stopping, the bearing surfaces are usually formed from very hard, wear resistant materials, such as those listed previously, and in particular: nitride steel, hard chromium or nickel plate, glass ceramics sintered alumina, beryllia, or synthetic sapphire. Design limitations and the high cost of fabricating forms for sintered preforms of the hard, refractory ceramic materials have resulted in the use of sprayed coatings of ceramics or cermet alloys. One of the best of these coatings is formed from chromium oxide. The coating is typically applied by hot-powder processes: the oxide of trivalent chromium is applied as a powder using a plasma arc torch or detonation gun, or it may be sputtered using RF energy in a low pressure inert gas. The resulting chromium oxide coating has great hardness, some resiliency and relatively high thermal conductivity.
Chrome oxide and most refractory ceramic coatings that are applied by hot powder processes are porous. Plasma-sprayed chrome oxide, for example, typically contains one to three percent void space or cavities. These cavities are useful as lubricant reservoirs and as storage for wear debris. However, the void volume at the contact surfaces increases the bearing clearance volume, so that the stiffness and bottoming capabilities of the operating bearing are reduced and the rotational velocity at setdown is increased. Also, the voids in ceramics surfaces may be of such dimensions that water and other condensable vapors in the ambient environment are sorbed in the capillaries. As a result, the bearing stiffness may change, even in a sealed environment. An additional detrimental effect of this capillary sorption is that condensed fluids such as water may form corrosion cells between conductive surfaces such as chromium oxide and active metal sublayers. Thus, as an example, etch-roughened beryllium metal was plasma-arc sprayed with a chromium oxide coating of 0.005 inches thickness (nominal) and the coating was exposed to 30 percent relative humidity, then 90 percent relative humidity. As a result, the amount of sorbed surface water increased by approximately 4 milligrams per square inch, and corrosion blisters resulted.
The reliability of devices that incorporate gas-lubricated bearings depends upon their start-stop life. Accordingly, some form of boundary lubricant is utilized in virtually all such devices to reduce friction and wear during starting and stopping. It may thus be appreciated that it is desirable to have an effective boundary lubricant that fills the void spaces and precludes corrosion in the metal substrates of refractory wear coatings.